Proton (‹See Tfd›Russian: протон) ('proton') was a Soviet series of four cosmic ray and elementary particle detecting satellites. Orbited 1965–68, three on test flights of the UR-500ICBM and one on a Proton-K rocket, all four satellites completed their missions successfully, the last reentering the Earth's atmosphere in 1969.
^"Proton 1". NASA Space Science Data Coordinated Archive. Retrieved 23 October 2020.
^"Proton 2". NASA Space Science Data Coordinated Archive. Retrieved 23 October 2020.
^"Proton 3". NASA Space Science Data Coordinated Archive. Retrieved 23 October 2020.
^"Proton 4". NASA Space Science Data Coordinated Archive. Retrieved 23 October 2020.